This invention relates to semiconductor switches and, more particularly, to a thermal protection arrangement for semiconductor switches such as used in motor control devices.
Motor control devices having controlled "soft start" and "soft stop" function wherein the motor is protected from overcurrent surges during both starting and starting operations are described within U.S. Pat. No. 5,341,080 entitled Apparatus and Three Phase Induction Motor Starting and Stopping Control Method.
Semiconductor switches such as silicone controlled rectifiers (SCR), when used within each pole of a multi-pole electric motor to control the starting and stopping function can become subjected to thermal damage when an attempt is made to re-start a motor while the SCR's are warm from previous operation. Motor control devices utilizing thermal enclosures for cooling the semiconductor switches used to control the motor start and stop functions are described in U.S. patent application Ser. No. 08/904,293, filed Jul. 31, 1997 entitled Compact Electrical Enclosure.
U.S. Pat. No. 3,854,631 entitled Motor Starter Unit, describes a lockout counter to prevent motor restart after a predetermined number of motor starts to protect the electric motor and associated electric equipment from thermal damage. Since the cool down rates for electric motor windings and SCR junctions are not the same, a predetermined start inhibit signal to protect motor windings within electric motors exterior to a motor controller cabinet may not be sufficient to prevent thermal damage to the SCR's used within the motor controller unit contained within the cabinet.
The use of thermal fluid transport devices in contact with semiconductor components is found within U.S. Pat. No. 4,848,090 entitled Apparatus for Controlling the Temperature of an Integrated Circuit Package and U.S. Pat. No. 5,034,688 entitled Temperature-Conditioning Support for Small Objects Such as Semiconductor Components and Thermal Regulation Process Using Said Support. Both of the aforementioned patents describe the use of a thermal sensing device used in feed-back relation to a fluid transport device to control the operating temperatures of the semiconductor components. The size of the fluid transport pipes and heat sink supports are of sufficient size to insure adequate cooling of the semiconductors when used within circuits containing induction motors.
The use of a fan assembly and heat sink support, per se, to control the temperature of semiconductor components is described within U.S. Pat. No. 5,690,468 entitled Fan Assembly for an Integrated Circuit. It is believed that the use of a fan assembly and heat sink support within motor control devices would require over-sized fans to provide sufficient air flow to meet the SCR cooling requirements, and that such fan assembly could introduce air-borne contaminants, such as dust, dirt and the like.